Semiconductor solar cells are utilized to convert light energy to useable electrical voltages and currents through the photovoltaic effect. Briefly, a typical semiconductor solar cell includes an interface between n-type and p-type transparent semiconductor materials. Light shining on the semiconductor materials adjacent the interface creates hole-electron pairs in addition to those otherwise present, and the minority charge carriers migrate across the interface in opposite directions. There is no compensating flow of majority carriers, so that a net electrical charge results. A useful electrical current is then obtained in an external electrical circuit by forming ohmic contacts to the materials on either side of the interface.
In general terms, a photovoltaic solar cell is fabricated by depositing or attaching the appropriate semiconductor layers onto a substrate, and then adding additional components to complete the cell. The individual solar cells are connected together into large arrays to deliver power of the desired voltage and current. The ratio of power output to area of the solar cell array is an important design parameter, since the required power output could in principle be satisfied, for example, by larger numbers of low power density solar cells made of silicon or by smaller numbers of high power density solar cells made of gallium arsenide. Large numbers of solar cells require more supporting structure and area with solar access (such as the scarce area on rooftops) adding cost and complexity to PV system, and reducing the amount of energy which can be generated on a given site, such as a building or plot of land.
A number of the individual solar cells are connected together in an array, typically by fastening the solar cells to a support structure and then electrically interconnecting the cells into series and parallel arrangements, as necessary to meet the power requirements. This incentive for improved power output and area reduction is particularly pressing for crystalline solar cells such as mono-crystalline silicone solar cells, which have higher power output per unit area than thin-film solar cells, but continue to be at a disadvantage in cost per unit area, because of their manufacturing requirements.
The power output for most solar cells decreases significantly with increased cell temperature, with about a 0.4-0.5% loss for every degree Celsius. This drop in output power is mainly due to the characteristic open circuit voltage, which decreases by about 0.4%/° C. from increased recombination in the semiconductor stemming from the greater prevalence of phonons at higher temperatures. Under standard conditions of one sun—1000 W/m2 solar illumination, the typical solar cell operating temperature may increase 30-40° C.—so this negative effect can cause a significant power loss of about 15-20%. During the winter (in northern climates) the PV panels are exposed to extreme temperature conditions, and potential snow and ice buildup. For these reasons, thermal management is an important consideration for any PV system. Nearly every negative mechanism (oxidation, delamination, encapsulation failure) is accelerated by high or low temperatures, sometimes exponentially with temperature.
U.S. Pat. No. 4,710,588 issued in Dec. 1, 1986 discloses a solar cell that generates an electrical voltage with contributions from both photovoltaic and thermoelectric effects, when a high thermal gradient is impressed across a semiconductor p/n solar cell. To achieve a substantial thermoelectric voltage contribution, the front side of the solar cell is heated to an elevated temperature consistent with efficient operation of the photovoltaic mechanism of the solar cell, and the back side of the solar cell is cooled to a lower temperature. Significantly, this configuration uses a discrete TE element physically attached to the back of a complete and discrete PV cell (contrast with the completely integrated PV-TE cell disclosed herein). The magnitude of the thermoelectric voltage contribution is increased by reducing the coefficient of thermal conductivity of the solar cell material, by using face electrodes having the proper thermoelectric potentials in contact with the solar cell material, by increasing the light intensity and then the heat input to the front side of the solar cell, and by cooling the back side of the solar cell.
A significant amount of development of Photovoltaic/Thermal (PV/T) modules is known. One provides a combination photovoltaic array and solar thermal water heater. A first problem with this configuration is the requirement for a nearby thermal heat requirement, such as heating water. By far, most PV installations are not associated with a heat requirement. Consider the many arrays mounted on office buildings, warehouses, or simply ground mounted. However, even if such a heat load is present, when the water heater component has stored all the hot water possible, such as during a day when there is no use, the water temperature is so high as to render its cooling effect on the photovoltaic module useless. In fact, the module can remain at high temperature when it would otherwise cool down with the evening ambient decline. Another problem with conventional photovoltaic/thermal is its focus on water heating, which can lead to significant temperature gradients across the array, with corresponding thermal stresses. Photovoltaic solar cells having a component for reducing heat to increase the output power have been limited to rejection of the photovoltaic heat to domestic or process water heating. Further problems with this type of cooling include the fact that the cooling effect is often negligible, allowing unacceptable thermal cycling stress on all components and that the thermal load requirements do not allow for optimum design of the electric generation system due to the variability of operating conditions.
Thus, there is a continuing need for an integrated hybrid solar cell that integrates photovoltaic and thermoelectric cell elements to increase the ratio of power output to area for solar cells and solar cell arrays and that operates in different ambient weather conditions with increased efficiency and longevity. The present invention fulfills this need, and further provides related advantages.